Not applicable.
This invention is directed to a novel configuration of fluid flow channels that provides for passive control of sample transport in microfluidic systems and particularly for controlling sample injection for miniature total analysis systems (xcexcTAS). The apparatus and method disclosed herein can be used to control sample transport in systems that employ electrophoretic, electrochromatographic, and pressure-driven flow.
Recent advances in miniaturization have led to the development of microfluidic systems that are designed, in part, to perform a multitude of chemical and physical processes on a micro-scale. Typical applications include analytical and medical instrumentation, and industrial process control equipment. In this context, there is a need for devices to provide very precise control over small flows as well as small volumes of liquid in microscale channels. A common method for introducing a sample for analysis into a microscale separation channel is by means of voltage switching at the intersection of two intersecting channels. This method is illustrated generally in FIG. 1. There are essentially two steps involved in a typical sample injection. In the first step (FIG. 1A), a voltage is applied along channel 110 to move a sample through the channel (sample introduction channel). In step two (FIG. 1B), a voltage is applied along channel 120, the separation channel. When the voltage is switched from channel 110 to channel 120 that portion of the sample occupying the intersection between the two channels is carried into the separation channel 120 for subsequent analysis. Ideally, the process of switching voltage between channels serves to inject a precise and reproducible quantity of sample into the separation channel. However, in practice such is often not the case.
Electroosmotic-driven fluid flow, such as discussed above, is a xe2x80x98potential flowxe2x80x99 which means that fluid flow follows the paths traced by the streamlines of the electric field. Leakage occurs in this injection scheme because fluid streamlines, which correspond to electric field lines in electroosmotic-driven flow, enter the electrically floating channel. This phenomenon is graphically illustrated in FIG. 2 which shows the electric field lines at the intersection between channel 110 having an electric field contained therein and one that is floating 120. It can be seen that the electric field lines intrude a significant distance into the floating channel. This intrusion of electric field lines into the electrically floating channel not only explains the xe2x80x9cleakagexe2x80x9d shown in FIG. 3 but also explains why the sample fluid is observed to enter microchannel 120 prior to application of a potential gradient to that microchannel and fluid continues to flow (leak) into microchannel 120 after the potential gradient has ceased to be applied. As can be readily appreciated, these processes can introduce additional and unknown quantities of sample into the separation channel, thereby causing the analysis to be inaccurate.
The problem of leakage in injection devices has been recognized and means for mitigating this problem have been proposed. Ramsey in U.S. Pat. No. 5,858,195 and Published PCT Application No. WO96/04547 and Parce in U.S. Pat. No. 5,885,470 employed a scheme called xe2x80x9ccontrolled electrokinetic material transportxe2x80x9d, to control cross-channel leakage in microchannel systems and particularly in arrangements of integrated microchannels. In this scheme separate electric potentials are applied across the various microchannels. However, these methods require careful control of multiple electrical power sources as well as a priori knowledge of the conductive properties of all fluids in all channels to determine the required voltages. Furthermore the method is susceptible to disruption due to variations in fluid compositions, hydrostatic pressure-driven interferences, and diffusion effects, all of which may degrade the quality or purity of the injected sample. Moreover, for each change in channel system design it is necessary to reconfigure, and perhaps refabricate, the electronics system to accommodate the design change. This is true even for changes that may not necessarily affect the general channel configuration and geometry but that change the electrochemical properties of the system, such as the conductivity or zeta potential. Moreover, this approach introduces a significant additional drain on the power supply.
Another approach to the problem of sample injection leakage has been described by Hasselbrink in U.S. patent application Ser. No. 09/669,862 xe2x80x9cMethod and Apparatus for Controlling Cross Contamination of Microfluid Channelsxe2x80x9d, assigned to the same assignee and incorporated herein by reference in its entirety. Hasselbrink provides method and apparatus for reducing or substantially eliminating channel cross-contamination, due to electric field streamlines entering the floating channel, hydrostatic pressure effects, and mass diffusion, during microfluidic sample injections by a reduction of the cross-sectional area of the fluid flow channels in proximity to the intersection. A non-orthogonal intersection microchannel geometry can also be used in conjunction with reduction in cross-sectional area to reduce the leakage of electric field lines away from the intersection during sample injection. However, this approach suffers from the need for more complex channel fabrication.
It can be desirable to provide for parallel separations schemes on a given sample. In this way, several different analyses can be performed simultaneously on the same sample by proper configuration of the separation channels. As before, a portion of the sample to be analyzed can be introduced into each of a plurality of parallel separation channels by switching the sample-driving voltage to carry the portion into each separation channel. However, with this approach two significant problems arise. Referring now to FIG. 4, a volume of sample can be trapped in the region 310 between the individual separation channels 120. Unlike the case for a single separation channel (cf. FIG. 2), application of reduced voltages along the sample channel 110 cannot sweep the sample from the interchannel region. However, as before, electrical field line penetration into the sample channel allows a continuous sample flow into the separation channel by diffusion.
Because the different separation channels provide for different analyses, the buffer solutions that flow in the different channels can have different compositions and thus different electrical conductivities, pH, compositions, etc. If the solutions from the different separation channels are allowed to mix the desirable plug-like flow associated with electrokinetic fluid pumping mechanism can be compromised degrading the separation performance of the device.
Prior art sample schemes for the injection of a sample volume into a separation channel have been illustrated by the application of an electric field to the separation channel. However, sample injection can also be accomplished by the application of a pressure. While the issue of sample leakage onto a single sample channel is not as critical for pressure injection as for electroosmotic flow injection schemes, problems with parallel channel separations discussed above remain.
Accordingly, the present invention is directed to apparatus and method for eliminating the undesirable effects of siphoning, xe2x80x9cdeadxe2x80x9d regions, and concentration gradients that are present in conventional microfluidic analytic devices by providing strategically placed fluid flow channels, or auxiliary channels, whose purpose is to provide alternate routes for sample transport that can be toward or away from channel intersections, i.e., passive injection control.
As discussed above, problems with injecting a sample from a sample channel to a separation channel in an electric field-driven microfluidic system are due, principally, to lack of control of electric field lines, i.e., propagation of electric field lines into regions where they produce deleterious effects. Prior means of control involved, inter alia, the use of multiple power supplies or complicated electronics to define the electric field at all points in a microfluidic system.
In its most basic embodiment, the present invention affords passive injection control of sample transport for both electric field-driven and pressure-driven systems by providing additional fluid flow channels or auxiliary channels disposed on either side of a sample separation column. The inlet ends of these auxiliary flow channels can be joined conveniently with the inlet end of the sample separation column proximate the point of sample injection. Similarly, the outlet ends of the side channels can be joined conveniently with the outlet end of the sample separation channel. The auxiliary flow channels are sized such that volumetric fluid flow rate through these channels, while sufficient to move the sample away from the sample injection region in a timely fashion, is less than that through the separation channel. In general, it is preferred that the ratio of fluid flow rate through the separation channel to that through the auxiliary channels be on the order of about 10:1. It will be appreciated that since channel volume is a product of the channel cross-sectional area and channel length, fluid flow through a channel can be affected by changes in either of these parameters s well as by packing the auxiliary flow channels with a packing material to reduce the fluid flow rate through these channels.